A percussion apparatus can comprise a stepped piston sliding in a body and delimiting with its bores a driving chamber subjected alternately, by means of a distributor, to the pressure prevailing in the feed circuit having a high-pressure accumulator, and to the return pressure of the apparatus, and an opposed annular chamber constantly connected to the feed pressure.
The distributor is actuated by hydraulic means according to the position of the percussion piston, for example by providing a control section which, depending on the position of the percussion piston, is subjected alternatively to the feed pressure of the apparatus or to that of the return circuit.
When the feed pressure is applied to the control section of the distributor, the driving chamber of the percussion piston is subjected to this same pressure and that the piston is then accelerated during it percussion travel and, conversely, when the return circuit pressure is applied to the control section of the distributor, the drive chamber of the percussion piston passes through its return travel.
It is understood that it would be possible to design an hydraulically controlled system which would function in the opposite way.
For reasons of design it is known that it is necessary to hold the distributor hydraulically in each position at the ends of its travel during the time separating the command pressure signals provided by the percussion piston and that it is also necessary to allow the distributor to cover its course systematically and completely in order to switch the circuits correctly; in short, the distributor must be "bi-stable".
This dual function may, for example, be achieved by known means by providing calibrated openings in the body of the distributor which permit either the filling of the control chamber of the distributor with fluid under pressure, or the emptying of it towards the return circuit.
However, for practical reasons, these openings, which cannot be connected simultaneously and constantly with the control chamber in order not to cancel their respective actions, are effective only over part of the displacement of the distributor.
It is therefore imperative that the communications established by the percussion piston, during its movement, between the control circuit of the distributor and alternatively the feed circuit and the return circuit of the apparatus should be maintained for a sufficient time to ensure the displacement of the distributor until the opening of the locking orifice.
The relative movements of the percussion piston and the distributor are of very great importance. The accelerations and speeds of movement of the distributor depending on the movement of the percussion piston, as well as the point of hydraulic control of the distributor by the piston during its descent will have to be selected carefully.
The main difficulties encountered arise during the travel of the percussion piston. It is, in fact, known from French patent 2,509,217, for example, that the kinetic energy delivered by the percussion piston to the tool at the moment of impact is partially transmitted to the rock an that the balance may be restored in the form of kinetic energy to the piston.
In this case the piston recoils and remains for a very short time in the vicinity of the tool. Now, as indicated previously, the connection effected by the percussion piston between the control circuit of the distributor and the return circuit must, in this case essentially last sufficiently long to permit the distributor to complete the part of its displacement that will allow it to reach the locking orifice of the following phase of the cycle.
Since the velocity of the piston on approaching the tool is great, the hydraulic switching effected by the piston will therefore have to be produced fairly early in the course of its descent to ensure that in the case of recoil it lasts sufficiently long. In fact, if the control point is close to the moment of the end of the descending travel of the piston, the duration of communications established by the latter is insufficient for the distributor to have had time, during its travel, to reverse the action of its locking orifices, the distributor will then cover only an insufficient part of its travel and will return prematurely to the starting point, remaining at the lock of the preceding phase of the cycle and will then keep the driving chamber of the piston at the pressure of the high-pressure feed circuit, which will have the result of re-accelerating the piston after its recoil and on a weak course, and a very weak second impact will possibly be produced, greatly disrupting the operational cycle of the apparatus.
Conversely, if the control point of the distributor is located too early in the descent of the piston, there is a risk that the distributor will prematurely cover its reverse travel and therefore that the drive chamber of the piston will be switched too early towards the low pressure.
Since this chamber is no longer fed by fluid under pressure, the piston will therefore no longer be accelerated during the end of its travel, and hence there will be a considerable loss of performance and the formation of a vacuum in the drive chamber, with the danger of cavitation if there is a recoil phenomenon.
Naturally, in the case of a percussion apparatus operating with a constant travel, a constant pressure and therefore a constant acceleration, it is known that it is possible to find a compromise and to determine an ideal distributor control point on the descent of the piston, the various operational parameters being fixed.
On the other hand, for an apparatus equipped with a course-variation or pressure-variation system having a fixed distributor control point, the rate of descent of the piston close to the impact varies to considerable extents and the time elapsing between the moment of control and the impact therefore also varies. It then becomes impossible to find a compromise for the position of the distributor control point, that which is correct for a high velocity of the piston being premature for a lower velocity, and if correct for a low piston velocity, being too late for a higher velocity.